1. Field of the Invention
The invention relates generally to the field of marine electromagnetic surveying. The present invention relates more specifically to a method for removing an undesirable response called the “airwave” from marine electromagnetic survey data.
2. Background Art
Porous subsurface sedimentary rock formations are typically saturated with fluids as a result of having been deposited in a body of water during sedimentation. As a result, the fluids were initially entirely water. In some subsurface formations the water in the pore spaces has been displaced to some extent after sedimentation by hydrocarbons such as oil and gas. Thus, in some present day subsurface formations, the fluids in their pore spaces may be water, gas or oil, or mixtures of the foregoing.
Detection of formations having less than fully water-saturated pore space, that is, when oil or gas may be present in the pore spaces, is of significant economic interest. Certain techniques for detection of such formations include determining existence of electrical resistivities in the subsurface that are anomalously high. The principle of such detection is based on the fact that the flow of electric current through a porous rock formation is related to the fractional volume of the pore spaces with respect to the total rock volume, the spatial configuration of the pore spaces and the electrical properties of the fluids filling the pore spaces. Brine-saturated porous rock formations, for example, are typically much less resistive than the same rock formations having hydrocarbons in some or all of the pore spaces, because brine is a relatively good electrical conductor while hydrocarbons are typically good electrical insulators.
Various techniques for measuring the electrical resistivity of subsurface rock formations are known in the art, for example, transient controlled source electromagnetic survey techniques such as described in International Patent Application Publication No. WO 03/023452, the contents of which are incorporated herein by reference. Such techniques in general include imparting an electromagnetic field into the subsurface and measuring electric and/or magnetic fields induced in the subsurface in response to the imparted electromagnetic field. For such measurement techniques, the electromagnetic field may be imparted using an electric field transmitter, for example, a device configured to pass an electric current through a dipole electrode. Alternatively a magnetic field transmitter may be used, for example, a device configured to pass an electric current through a wire loop or a plurality of such loops. The receivers used to detect the responsive electromagnetic fields may be, for example, dipole electrodes for measuring potential differences (electric field potential), or may be wire loops, pluralities of wire loops or magnetometers for measuring magnetic field amplitude and/or the time derivatives of magnetic field amplitude.
In transient controlled source electromagnetic surveying, the electric current passed through the transmitter to impart the electromagnetic field may be controlled to provide one or more step changes in the current amplitude. Step change in the transmitter current induces what are referred to as “transient” electromagnetic fields, and the responses measured by the receivers are related to transient response of the formations in the earth's subsurface. Step change in the transmitter current may be obtained by switching the current on, switching the current off, reversing polarity, or combinations of the foregoing. A particularly advantageous form of transmitter current switching configuration used to impart a controlled source electromagnetic field is a so called “pseudo-random binary sequence” (PRBS).
A typical marine electromagnetic survey system includes a controllable current source disposed on a survey vessel or auxiliary vessel. The vessel tows a transmitter cable on or near the water bottom for transmitting an electromagnetic field into the formations below the water bottom by passing transient current between two electrodes disposed on the transmitter. The electromagnetic response of the system including the water and the formations below the water bottom is measured by receivers disposed on a cable on or near the water bottom. The receivers may be pairs of electrodes configured such that each receiver measures the potential difference between its pair of electrodes. All the electrodes are typically in the same vertical plane. In some survey systems, different vessels may be used to tow the transmitter and the receivers to enable the transmitter-receiver separation (“offset”) to be more readily adjusted. As described in the WO 03/023452 publication referred to above, the signals in the receivers are measured, as well as the signal transmitted by the transmitter. By deconvolving the measured receiver signal with the measured transmitter signal, the impulse response of the subsurface for the particular transmitter-receiver configuration is obtained.
As a practical matter, the electromagnetic signal generated by the transmitter can follow three general transmission paths to the receiver(s), these paths being through the formations below the water bottom, through the water layer itself and through the air above the water layer. In deep water, for example 2 km or more, and wherein the transmitter and receivers are disposed near the water bottom as is typical in known survey techniques, the part of the signal propagating through the air has a negligible impact on the signals detected by the receiver(s) because the electromagnetic signal from the transmitter is substantially attenuated and delayed in time from the time of the transient current event as it moves to the water surface and returns to the receiver(s) on the water bottom. By contrast, in shallow water, for example 100 m or less, the portion of the signal that moves through the water and air is substantial with respect to the total measured signal. Shallow water electromagnetic surveying has been thought to be impracticable as a result.
There is a need for techniques to determine electromagnetic response of the subsurface formations below the water bottom given the presence of the air wave.